snapshot-testing

Snapshot Testing with Verify

When to Use This Skill

Use snapshot testing when:

• Verifying rendered output (HTML emails, reports, generated code)
• Approving public API surfaces for breaking change detection
• Testing HTTP response bodies and headers
• Validating serialization output
• Catching unintended changes in complex objects

---

What is Snapshot Testing?

Snapshot testing captures output and compares it against a human-approved baseline:

1. First run: Test generates a .received. file with actual output
2. Human review: Developer approves it, creating a .verified. file
3. Subsequent runs: Test compares output against .verified. file
4. Changes detected: Test fails, diff tool shows differences for review

This catches unintended changes while allowing intentional changes through explicit approval.

---

Installation

Add Verify Package

dotnet add package Verify.Xunit
# or for other test frameworks:
dotnet add package Verify.NUnit
dotnet add package Verify.MSTest

Configure ModuleInitializer

Create a ModuleInitializer.cs in your test project:

using System.Runtime.CompilerServices;

public static class ModuleInitializer
{
    [ModuleInitializer]
    public static void Init()
    {
        // Use source-file-relative paths for verified files
        VerifyBase.UseProjectRelativeDirectory("Snapshots");

        // Configure diff tool (optional - auto-detected)
        // DiffTools.UseOrder(DiffTool.Rider, DiffTool.VisualStudioCode);
    }
}

---

Basic Usage

Simple Object Verification

[Fact]
public Task VerifyUserDto()
{
    var user = new UserDto(
        Id: "user-123",
        Name: "John Doe",
        Email: "john@example.com",
        CreatedAt: new DateTime(2025, 1, 15));

    return Verify(user);
}

Creates VerifyUserDto.verified.txt:

{
  Id: user-123,
  Name: John Doe,
  Email: john@example.com,
  CreatedAt: 2025-01-15T00:00:00
}

String/HTML Verification

[Fact]
public async Task VerifyRenderedEmail()
{
    var html = await _emailRenderer.RenderAsync("Welcome", new { Name = "John" });

    // Use extension parameter for proper file naming
    await Verify(html, extension: "html");
}

Creates VerifyRenderedEmail.verified.html - viewable in browser.

---

Email Template Testing

Use Verify to catch unintended changes in rendered email templates:

[Fact]
public async Task UserSignupInvitation_RendersCorrectly()
{
    var renderer = _services.GetRequiredService<IMjmlTemplateRenderer>();

    var variables = new Dictionary<string, string>
    {
        { "OrganizationName", "Acme Corporation" },
        { "InviteeName", "John Doe" },
        { "InviterName", "Jane Admin" },
        { "InvitationLink", "https://example.com/invite/abc123" },
        { "ExpirationDate", "December 31, 2025" }
    };

    var html = await renderer.RenderTemplateAsync(
        "UserInvitations/UserSignupInvitation",
        variables);

    await Verify(html, extension: "html");
}

Benefits for email testing:

• Catches CSS/layout regressions
• Detects broken template variables
• Visual review in diff tool
• Version control tracks email changes

---

API Surface Approval

Prevent accidental breaking changes to public APIs:

[Fact]
public Task ApprovePublicApi()
{
    var assembly = typeof(MyLibrary.PublicClass).Assembly;

    var publicApi = assembly.GetExportedTypes()
        .OrderBy(t => t.FullName)
        .Select(t => new
        {
            Type = t.FullName,
            Members = t.GetMembers(BindingFlags.Public | BindingFlags.Instance | BindingFlags.Static)
                .Where(m => m.DeclaringType == t)
                .OrderBy(m => m.Name)
                .Select(m => m.ToString())
        });

    return Verify(publicApi);
}

Or use the dedicated ApiApprover package:

dotnet add package PublicApiGenerator
dotnet add package Verify.Xunit

[Fact]
public Task ApproveApi()
{
    var api = typeof(MyPublicClass).Assembly.GeneratePublicApi();
    return Verify(api);
}

Creates .verified.txt with full API surface - any change requires explicit approval.

---

HTTP Response Testing

[Fact]
public async Task GetUser_ReturnsExpectedResponse()
{
    var client = _factory.CreateClient();

    var response = await client.GetAsync("/api/users/123");

    // Verify status, headers, and body together
    await Verify(new
    {
        StatusCode = response.StatusCode,
        Headers = response.Headers
            .Where(h => h.Key.StartsWith("X-"))  // Custom headers only
            .ToDictionary(h => h.Key, h => h.Value.First()),
        Body = await response.Content.ReadAsStringAsync()
    });
}

---

Scrubbing Dynamic Values

Handle timestamps, GUIDs, and other dynamic content:

[Fact]
public Task VerifyOrder()
{
    var order = new Order
    {
        Id = Guid.NewGuid(),  // Different every run
        CreatedAt = DateTime.UtcNow,  // Different every run
        Total = 99.99m
    };

    return Verify(order)
        .ScrubMember("Id")  // Replace with placeholder
        .ScrubMember("CreatedAt");
}

Output:

{
  Id: Guid_1,
  CreatedAt: DateTime_1,
  Total: 99.99
}

Global Scrubbing

Configure in ModuleInitializer:

[ModuleInitializer]
public static void Init()
{
    VerifierSettings.ScrubMembersWithType<DateTime>();
    VerifierSettings.ScrubMembersWithType<DateTimeOffset>();
    VerifierSettings.ScrubMembersWithType<Guid>();

    // Scrub specific patterns
    VerifierSettings.AddScrubber(s =>
        Regex.Replace(s, @"token=[a-zA-Z0-9]+", "token=SCRUBBED"));
}

---

File Organization

Recommended Structure

tests/
  MyApp.Tests/
    Snapshots/           # All verified files
      EmailTests/
        WelcomeEmail.verified.html
        PasswordReset.verified.html
      ApiTests/
        GetUser.verified.txt
    EmailTests.cs
    ApiTests.cs
    ModuleInitializer.cs

.gitignore

# Verify - ignore received files (only commit verified)
*.received.*

.gitattributes

# Treat verified files as generated (collapse in PR diffs)
*.verified.txt linguist-generated=true
*.verified.html linguist-generated=true
*.verified.json linguist-generated=true

---

CI/CD Integration

Fail on Missing Verified Files

[ModuleInitializer]
public static void Init()
{
    // In CI, fail instead of launching diff tool
    if (Environment.GetEnvironmentVariable("CI") == "true")
    {
        VerifyDiffPlex.UseDiffPlex(OutputType.Minimal);
        DiffRunner.Disabled = true;
    }
}

GitHub Actions

- name: Run tests
  run: dotnet test
  env:
    CI: true

- name: Upload snapshots on failure
  if: failure()
  uses: actions/upload-artifact@v4
  with:
    name: snapshots
    path: |
      **/*.received.*
      **/*.verified.*

---

When to Use Snapshot Testing

---

Best Practices

DO

// Use descriptive test names - they become file names
[Fact]
public Task UserRegistration_WithValidData_ReturnsConfirmation()

// Scrub dynamic values consistently
VerifierSettings.ScrubMembersWithType<Guid>();

// Use extension parameter for non-text content
await Verify(html, extension: "html");

// Keep verified files in source control
git add *.verified.*

DON'T

// Don't verify random/dynamic data without scrubbing
var order = new Order { Id = Guid.NewGuid() };  // Fails every run!
await Verify(order);

// Don't commit .received files
git add *.received.*  // Wrong!

// Don't use for simple assertions
await Verify(result.Count);  // Just use Assert.Equal(5, result.Count)

---

Integration with MJML Email Testing

See the aspnetcore/transactional-emails skill for the complete pattern:

1. MJML templates with {{variable}} placeholders
2. Render to HTML with test data
3. Snapshot test the rendered output
4. Review changes in diff tool before approving

This catches:

• Broken variable substitution
• CSS/layout regressions
• Email client compatibility issues
• Unintended content changes

---

Resources

• Verify GitHub: https://github.com/VerifyTests/Verify
• Verify.Xunit: https://github.com/VerifyTests/Verify.Xunit
• ApiApprover: https://github.com/JakeGinnivan/ApiApprover
• DiffPlex Integration: https://github.com/VerifyTests/Verify.DiffPlex